Cystic Fibrosis (CF) is a common genetic disorder characterized by defective epithelial chloride transport and progressive, lethal lung disease. Chronic sinusitis, which plagues approximately 50% of CF patients, is treated at Stanford with a combination of maxillary antrostomy placement followed by monthly maxillary sinus tobramycin lavage. A total of four patients have been enrolled in this protocol, an evaluation of maxillary sinus treatments in patients with antrostomies. This randomized, double blind comparison of maxillary sinus tobramycin lavage versus placebo is designed to document the effectiveness of tobramycin lavage and determine the rate of sinusitis recurrence. Nasal mucosa tranepithelial potential differences are similar to maxillary sinus transepithelial potential differences, except that maxillary sinus tranepithelial potential differences, during baseline determinations, are more sensitive to the effects of local chronic infection.